1. Technical Field
The present invention relates to a lead-free X-ray shielding rubber composite made of rare earth mixed with other metals and compounds.
2. Description of Related Art
The present invention discloses an X-ray shielding polymer composite featuring high shielding property, lead-free, light and soft material, good physical and mechanical property. It can be applied in the medical diagnostic X-ray machine, X-ray diffractmeter, the transmitter of the electron microscopy and the protection of the staff working with X-rays.
The development, production, promotion and application of the radiation protection material are some of the key issues on civilian hygiene protection, and also the important components of radiation protection in the nuclear energy development field and in the military field. Among the ray workers in our country, the medical diagnosis & cardiovascular Intervention treatment (X-ray machine and CT machine) X-ray workers have long been under the worst protection conditions. They need a large quantity of good quality and lightweight anti-radiation clothing & equipment. In the 21st century, the dramatic development of various fundamental researches and industrial technology developments around the nuclear energy and ray application (e.g. nuclear power station) puts forward a higher requirement for the shielding and personal protection in the fixed installations, such as buildings and equipment. The radiation protection material is also applied in the aerospace field because various moon exploration (Mars) & moon landing space probes, astronaut in-cabin clothes and astronaut out-of-cabin clothes for space walks all need high-efficient radiation protection.
The traditional radiation shielding materials mainly are made from composites of polymer matrix and lead or its compounds which act as the radiation absorbing components. The atom No. of Lead is 82. It has good energy absorption properties and high attenuation ability towards the low-energy X-ray photon, high-energy X-ray photon and γ-ray photon. However, the lead harm of the lead-contained radiation protection material shall not be ignored. As an important material in the radiation protection field, lead and its compounds have considerable demand. In China, the consumption of lead and its compounds only for medical protection clothes & equipment reaches 1000 t/a while the demand for lead and its compounds for other items, such as protection lead wall, lead plate, lead glass, lead coating and protection building materials, even reaches more than 10 thousand tons. Once these products exceed the service lives, their waste may seriously affect human health and the living environment. In order to realize the shielding effect, the protection product usually has a larger proportion of lead content, for example, if the weight of a piece of protection clothing is about 7.5 kg, then 80% of its weight comes from lead, so the harm caused by the high concentration of lead is self-evident. Lead may enter the human body through various ways, thus causing harm to the users. The waste products beyond the service life are generally disposed by landfill, thus the high-concentration lead gradually infiltrates the soil and pollutes the water. More seriously, the production environment of the lead-contained material is very harsh. Under the violent cutting and rotation of the mechanical equipment, the powder from lead and its compounds flies everywhere, so it is easy to attach to the eyes, nose and mouth, and enter the respiratory and digestive systems. Therefore, in such operational environment, it is difficult to avoid the intrusion of lead even with the best protective measures.
In terms of the radiation shielding property, lead also has defects. The absorption edge for K electron shell of the lead is 88 keV and this element has good absorption capacity towards the radiation with the energies ≧88 keV. Meanwhile, the absorption edge of the lead for L electron shell is 13 keV and has certain ray absorbing ability towards the radiation with the energy ≧13 keV, but the absorbing ability weakens quickly as the radiation energy increases, and when the radiation energy increases to 40 keV, the absorbing ability of the lead for L electron shell becomes very weak. Therefore, this energy region is named the “lead feeble absorbing area”. The abovementioned result indicates that the traditional lead containing shielding material has weak absorption capacity towards the radiation with energies between 40-88 keV, but the X-ray energy generated by the medical tube voltage commonly lower than 130 kVp is in the energy range of 40-88 keV, so obvious defects exist by regarding lead as the X-ray absorption matter in this energy region.
The research on the lead-free radiation shielding material is increasingly attracting much interest, for example, some elements, like Cu, Sn, Sb, I, Ba and so on, have been looked for to substitute lead. Some new technologies have become patents and some new products have been launched. However, from the perspective of the present lead-free shielding material, these substitutes are still not satisfactory for replacing the traditional Pb contained shielding materials. In view of Cu, Sn, Sb, I, Ba commonly used in the lead-free materials at present, the absorption energies for the K electron shell of Cu, Sn, Sb and I are 9.0, 29.2, 30.5 and 33.1 keV, respectively, far away from the lower limit (40 keV) of the “lead feeble absorbing area” (40-88 keV), thus it is impossible to remedy for the Pb weak absorptive region. Barium is very active chemically. So the barium existed in the form of compounds instead of simple substance Among the Ba compounds, BaO has the highest Ba weight content), but BaO has strong alkali, high chemical toxicity and expensive price, thus reducing its utilization value.
To address the problem above and realize lead-free and high radiation shielding effect, the present invention puts forward: replace the lead with the rare earth mixture, meanwhile add tin & its compounds, bismuth & its compounds and tungsten & its compounds in the mixture as the main shielding material, and then compound with polymer matrix (rubber) to prepare the material realizes all radiation produced at the tube voltage of 40-170 kVp shielding (shielding property of the material with 2 mm thickness is the same as that of the pure lead plate with 0.5 mm thickness or higher in the different tube voltage as mentioned above, which is the commonchinese & international standard). The reasons for selecting the mixed rare earths as the main radiation absorbing components are as follows: □Lanthanide has unique electronic structure and numerous transition modes involving the 5d and 4f electron shells, so the absorbed X-ray energy may dissipate with the electrons transition among different energy levels. □ The absorption energy of K shell electron of each element of the lanthanide series rises as the atom No. increases, that is, rises from the 38.9 keV of La to the 63.3 keV of Lu, thus the energy absorption area of mixed lanthanides would cover the 40-88 key of the ray radiation.
Therefore, they can efficiently remedy the weak absorption region of lead. In addition, the radiation shielding composite containing with high atom No. & its compounds (bismuth & its compounds and/or metal tungsten & its compounds) may further improve the shielding capacity of the high-energy range (X-ray produced at the tube voltage of 130-170 kVp), while adding low atom No. element & its compounds (tin & its compounds) may improve the shielding capacity of the low-energy range (mainly refer to X-ray beams produced at 40-80 kVp), thus the lead-free composite with full shielding of the X-ray beams produced at 40-170 kVp were obtained.
In the present invention, the lanthanide series used in shielding the important energy range of x-ray produced at 70-130 kVp and bismuth used in shielding another important energy range of x-ray produced at 130-170 kVp are organically modified via the introduction of the unsaturated ligand with double bonds, in this way, to realize the fine dispersion and good interfacial compatibility of the shielding component into polymer matrix so as to improve the x-ray shielding property, physical and mechanical properties of the composite.
Russian Patent RU2054439 and RU2028331 disclose a rubber anti-X-ray composite filled with the inorganic rare earth oxides, but due to the poor compatibility between the inorganic rare earths (rare earth oxides) and rubber matrix, it is difficult to finely and homogeneously disperse the fillers in to the polymer matrix. And for this reason, it is easy to form non-filled rubber areas where the probability of interaction between the dispersed phase of shielding elements and x-ray is much lower. This kind of areas are easy to be passed through by the radiation of the high energy ray, and limit the shielding performance of the composite. Besides, we find that direct addition of a large amount of the inorganic rare earth and metal bismuth (and its inorganic compounds) in the polymer matrix may greatly reduce the comprehensive property of the composite.
For this purpose, the present invention, when using lanthanide elements and bismuth element, adopts the combination of the inorganic compound and unsaturated organic complex of the two kinds of metal elements. Take the rare earth as an example, in order to reach high shielding, high dispersion and high property, divide the rare earth into two parts with one part being an inorganic compound of the rare earth via surface modification compounding with the polymer matrix and the other part being unsaturated organic complex of the rare earth compounding with the polymer matrix via in-situ reaction. When using the bismuth, the situation is the same with the rare earth.
The in-situ reaction principles of the present invention are: design and synthesize the metal organic salt with the unsaturated bonds having reactivity, compound it with the polymer matrix and add appropriate quantity of free radical initiator (e.g. peroxide) at the same time. The free radical decomposed and produced by the peroxide at the high-temperature environment of the composite preparation leads to the self polymerization of the metal unsaturated organic complex monomer dissolved in the matrix and also caused the rubber matrix to cross-link, wherein this poly-complexes have a poorer compatibility with the matrix than that of the unsaturated organic complex monomer, easy to precipitate from the matrix and aggregate to generate the poly-complex nano-particles (40-100 nm), at this time, the monomer concentration of the metal organic complex in the matrix decreases. The reduction of monomers in the matrix arising from the in situ reaction destroys the diffusion equilibrium, and causes more monomers to diffuse into matrix from the surface of the original dispersed crystal particles aggregated by rare earth organic complexes. The process of dissolving and diffusing will guarantee that the in-situ polymerization continues till all the complex radicals are consumed. The implementation of the in-situ reaction makes the dispersed phase of the shielding elements form nano-micron-level dispersed particles in the matrix and forms a strong chemical bonding structure between fillers and matrix. The composites prepared by in-situ reaction possess the excellent X-ray shielding property of the rare earth and other elements, and good conventional physical and mechanical property of the matrix polymer material, which realizes the high amount & high dispersion of the shielding fillers and the high strength & high shielding properties of the composite.